Window Data Layer for Detection Finetuning

This is my integration of the window data layer authored by Ross Girshick as part of R-CNN [1]. This data layer is useful for the finetuning of detectors by training on window crops instead of whole images.

This change was meant for release 0.99 and will be retroactively added to the release once merged.

[1] Ross Girshick, Jeff Donahue, Trevor Darrell, Jitendra Malik. Rich feature hierarchies for accurate object detection and semantic segmentation. [Arxiv 2013](http://arxiv.org/abs/1311.2524).